A natural division between the eastern and western slopes of the San Juans is the north-south line that runs approximately through Lake City, south of the crossing of the Piedra River by US Highway 160. In this super guidebook, twenty-seven contributors—all experts in their fields—artfully bring the geology, hydrology, animal and plant life, human histories, and travel routes of these eastern slopes to life. Designed to inform researchers, educators, and students about the region's complex systems, The Eastern San Juan Mountains also serves as an informative guidebook to accompany visitors along their travels on the Silver Thread National Scenic Byway, which stretches between South Fork and Lake City.
The Eastern San Juan Mountains deserves a place next to The Western San Juan Mountains on the bookshelf of every naturalist, researcher, resident, educator, student, and tourist seeking a greater understanding of this marvelous place and its history.
A natural division between the eastern and western slopes of the San Juans is the north-south line that runs approximately through Lake City, south of the crossing of the Piedra River by US Highway 160. In this super guidebook, twenty-seven contributors—all experts in their fields—artfully bring the geology, hydrology, animal and plant life, human histories, and travel routes of these eastern slopes to life. Designed to inform researchers, educators, and students about the region's complex systems, The Eastern San Juan Mountains also serves as an informative guidebook to accompany visitors along their travels on the Silver Thread National Scenic Byway, which stretches between South Fork and Lake City.
The Eastern San Juan Mountains deserves a place next to The Western San Juan Mountains on the bookshelf of every naturalist, researcher, resident, educator, student, and tourist seeking a greater understanding of this marvelous place and its history.
The Eastern San Juan Mountains: Their Ecology, Geology, and Human History
416
The Eastern San Juan Mountains: Their Ecology, Geology, and Human History
416Paperback
-
PICK UP IN STORECheck Availability at Nearby Stores
Available within 2 business hours
Related collections and offers
Overview
A natural division between the eastern and western slopes of the San Juans is the north-south line that runs approximately through Lake City, south of the crossing of the Piedra River by US Highway 160. In this super guidebook, twenty-seven contributors—all experts in their fields—artfully bring the geology, hydrology, animal and plant life, human histories, and travel routes of these eastern slopes to life. Designed to inform researchers, educators, and students about the region's complex systems, The Eastern San Juan Mountains also serves as an informative guidebook to accompany visitors along their travels on the Silver Thread National Scenic Byway, which stretches between South Fork and Lake City.
The Eastern San Juan Mountains deserves a place next to The Western San Juan Mountains on the bookshelf of every naturalist, researcher, resident, educator, student, and tourist seeking a greater understanding of this marvelous place and its history.
Product Details
| ISBN-13: | 9781607320845 |
|---|---|
| Publisher: | University Press of Colorado |
| Publication date: | 09/01/2011 |
| Pages: | 416 |
| Sales rank: | 1,077,359 |
| Product dimensions: | 6.10(w) x 8.80(h) x 0.91(d) |
About the Author
Read an Excerpt
The Eastern San Juan Mountains
Their Geology, Ecology, and Human History
By Rob Blair, George Bracksieck
University Press of Colorado
Copyright © 2011 University Press of ColoradoAll rights reserved.
ISBN: 978-1-60732-084-5
CHAPTER 1
A Legacy of Mountains Past and Present in the San Juan Region
David A. Gonzales and Karl E. Karlstrom
THROUGHOUT TIME, PEOPLE HAVE BEEN DRAWN TO MOUNTAINS for inspiration, recreation, and scientific exploration. Mountains are also vast warehouses of natural resources and libraries of geologic history.
Mountains form in response to the dynamic forces of our planet. The life spans of mountain belts, from initial uplifts to erosion to base levels, run from tens of millions to hundreds of millions of years. Ancient and active mountain belts are part of the fabric of continental crust and provide clues to events that build and reshape continents. The concept of plate tectonics (Condie 1989) provides a framework within which to investigate and explain mountain building. (For a summary of plate tectonics, refer to The Western San Juan Mountains,chapter 2.)
The San Juan Mountains are part of the extensive Southern Rocky Mountains (figure 1.1) and are dominated by some of the highest and most jagged summits in the continental United States. The San Juans reveal a fascinating geologic story of the creation and demise of many mountain ranges in this region during the past 1.8 billion years, including probable current uplift from active mountain-building processes.
The history of any mountain belt is deciphered from its modern landscape and the remaining rock record. An understanding of the geologic evolution of the San Juan Mountains comes from many studies done over the past 125 years. Field studies of rocks exposed in the region are supported by satellite images, geophysical probes of the deep earth, measurements of rock ages, and analyses of rock compositions. These data give geologists the ability to reconstruct the geologic history in the San Juan Mountains over a span of nearly two billion years, almost half the age of the Earth.
The modern San Juan Mountains preserve a history of many geologic events. In this chapter, we provide a basic overview of our understanding of these events within the context of models geologists have proposed.
ASSEMBLY OF A CONTINENT
Evidence of the oldest mountain-building events in the San Juan Mountains is preserved in ancient rocks that are exposed mostly in the Needle Mountains, along the southwestern edge of the San Juan Volcanic Field (plate 1), and on some of the area's isolated peaks and in some of its deeply eroded canyons. The Precambrian rocks in the San Juan Mountains were formed during ancient mountain-building events, between 1.8 and 1.0 billion years ago (Ga). Long ago, these rocks were eroded and beveled to sea level, leaving only their contorted mid-crustal roots to be studied. In the discussion that follows, we use the present location of North America as a reference point, although the continents have shifted positions over time and North America's old rocks did not form at the latitudes and longitudes at which they are now exposed.
The continental crust that forms the foundation of the San Juan Mountains was assembled in a series of continent-scale interactions along the previous edge of North America, during the early to middle Proterozoic (plate 2). Formation of these oldest mountains involved landmass collisions accompanied by magmatism and widespread contortion of rocks. This hypothesized scenario is similar to the tectonic processes active in modern Indonesia. In Indonesia, the Australian plate, which is analogous to the continental nucleus of proto–North America, is colliding with numerous volcanic-arc terranes (e.g., New Guinea and Sumatra), all of which are being sutured together to form a larger continental mass.
It is hypothesized that prior to 1.8 Ga, an ancient landmass known as Laurentia had been assembled by the accretion of rocks older than 2.5 Ga in a series of regional tectonic events (Van Schmus et al. 1993) (plate 2). The margin of this landmass was located along the present-day Colorado-Wyoming border. A volume of evidence indicates that between 1.8 and 1.7 Ga, large tracts of new continental crust were generated in subduction zones on or near the edge of Laurentia. This new crust collided with and was added to the margin of Laurentia in one or more regional tectonic events, forming the crustal foundation of Colorado and the Four Corners region (plate 2) (Gonzales and Van Schmus 2007; Karlstrom et al. 2004;Whitmeyer and Karlstrom 2007).
An alternative hypothesis to explain the creation of ancient crust in Colorado has been proposed in the past ten years (Bickford and Hill 2006; Hill and Bickford 2001). In this model, the edge of Laurentia, consisting mostly of rocks that are now ~ 1.85 Ga, extended into what is now central Colorado. This margin of Laurentia then underwent extensive rifting and magmatism between 1.8 and 1.7 Ga, producing large volumes of mafic-to-felsic volcanic and plutonic rocks. The key point in this model is that the edge of Laurentia was recycled and modified by later tectonic and magmatic events rather than composed of new additions of Proterozoic magmatic-arc crust.
In the Needle Mountains of southwestern Colorado (plate 2), the Irving Formation and Twilight Gneiss are interpreted as the remnants of ancient amalgamated oceanic volcanic-arc mountains that formed between 1.8 and 1.75 Ga (Barker 1969; Gonzales 1997; Gonzales and Van Schmus 2007). This block of crust was added to Laurentia between 1.73 and 1.7 Ga in a regional tectonic event known as the Yavapai orogeny (Karlstrom et al. 2004) (plate 2).
Following erosion and beveling of ancient volcanic-arc mountains in what is now southwestern Colorado, the region was covered by marine and river deposits sometime after 1.69 Ga. Deposited in tectonic basins that developed during continued regional convergence and compression (Karlstrom et al. 2004), these rocks were later buried during thrusting events and metamorphosed.
The period between 1.4 and 1.0 Ga was marked by episodes of magma generation over the present-day region of the San Juan Mountains. These events involved the emplacement of large masses of mostly granitic magma beneath the margin of Laurentia and were possibly related to renewed convergent tectonic events (subduction or continent collision). The granitic rocks formed during these events have been eroded into some of the majestic and rugged peaks in the Needle Mountains, such as the 14,000-foot Eolus, Sunlight, and Windom Peaks.
Regionally, tectonic events between 1.8 and 1.0 Ga in the San Juan Mountain region (plate 2) generated and modified crustal provinces over an extended period (Gonzales 1997; Gonzales and Van Schmus 2007). Mountain building during this period was a product of the assembly of crustal blocks along the entire edge of Laurentia. These events are collectively responsible for the formation of the crust beneath Colorado and are evident only in the vestiges of the now-eroded Precambrian mountain belts. The crustal-scale fabrics (i.e., faults, folds, and weak zones) created in these ancient mountain-building events, however, appear to have had important controls on the location and formation of many younger events, such as magmatism and mineralization along the Colorado Mineral Belt (Karlstrom et al. 2005; McCoy et al. 2005a).
THE ANCESTRAL ROCKY MOUNTAINS
Geologists have established many lines of evidence to indicate that, starting about 300 Ma, all of the world's major landmasses were assembled into a supercontinent called Pangaea (plate 3). Formation of this supercontinent is hypothesized as a series of major plate collisions that occurred between two large continental masses — the Northern Hemisphere continent, Laurasia (Laurussia in plate 3), and the Southern Hemisphere continent, Gondwana. Laurasia includes modernday Europe, Asia exclusive of the Indian subcontinent, Greenland, and North America. Gondwana includes modern-day South America, India, Australia, Africa-Arabia, and Antarctica.
The collision of Laurasia and Gondwana progressed from what is now north to south, along what is now the East Coast of North America, causing the uplift and formation of North America's Appalachians and Europe's Caledonian Mountains during the early phases of collision. Stresses from the collisions were transferred throughout the North American Plate. This led to uplift of blocks of crust in a currently northwest-southeast pattern, throughout the region that makes up present-day central and southwestern Colorado and southeastern Utah (plates 3 and 4). This created an ancient mountain range known as the Ancestral Rocky Mountains. These mountains were formed largely by uplift of large blocks of mountains, relative to other large blocks (deep sedimentary basins), along deep, near-vertical faults and, in places, thrust faults.
Remnants and evidence of the Ancestral Rocky Mountain uplifts are found in southwestern Colorado, as well as in Colorado's Front, Elk, and Sangre de Cristo Ranges. Numerous fractures and faults throughout the San Juan Mountains trend roughly parallel to the structural fabrics created during the uplift and development of the Ancestral Rocky Mountains. For example, the Snowdon Mountain block, in the Grenadier Range, was persistently a high area because of the resistant metamorphosed quartz-rich rocks exposed in this fault block.
The main evidence for the Ancestral Rocky Mountains, however, comes not from the mountains, because they were rather quickly eroded and covered by deposits of Permian–Jurassic age. Nor does it come from the faults, which tended to be reactivated and obscured by later mountain-building events. Instead, the best evidence comes from the very thick sedimentary deposits that filled basins adjacent to the uplifts (plate 4). As the fault-bound blocks of the Ancestral Rocky Mountains rose, erosion incised and wore away these highlands. This generated a vast accumulation of river and lake deposits in basins that flanked the highlands, such as the Paradox Basin. These deposits are preserved in rocks that are exposed in the region, such as the maroon-colored rocks of the Cutler Formation. The basins accumulated sediments near sea level, so these mountains might have been like modern-day mountainous areas of Greece, Italy, and the Aegean Sea — rugged uplifts cored by basement rocks but not as high above sea level as the present Rockies.
THE LARAMIDE ROCKY MOUNTAINS
During the Cretaceous, southwestern Colorado was covered by a vast inland seaway (plate 5). This seaway began a retreat as the Four Corners region once again underwent uplift, starting about 80 Ma and extending to perhaps 36 Ma (Cather 2004). During this pulse of mountain building, the Laramide orogeny, the western United States was compressed and uplifted. In the Southwest, regional deformation of the crust caused broad uplift of the Colorado Plateau and an extensive north-south belt of ranges that form the Laramide Rocky Mountains. The Rocky Mountain belt attained some of its present elevation during the Laramide orogeny, although these mountains were extensively eroded before about 35 Ma and have experienced later periods of uplift.
In the Southern Rocky Mountains (figure 1.1), the Laramide orogeny created a series of mountain ranges and related structural sedimentary basins. In many areas, the mountain units are bound by upturned sedimentary rocks and bordered by steep reverse and thrust faults. The San Juan Mountains rest on part of the fault-truncated Sangre de Cristo Range to the east, the Sawatch Range to the north, and the San Juan Uplift (Needle Mountains Uplift) to the south (plate 1).
Along the southern edge of the San Juan Mountains, the Needle Mountains block was uplifted during the Laramide, as a result of compression and shortening, to form a domal structure that is elongated to the northeast. Cather (2004) discusses current ideas regarding the structural evolution of the San Juan Uplift and San Juan Basin. He summarizes recent evidence for dextral-transpressive deformation along the eastern margin of the Colorado Plateau in the development of Laramide uplifts and basins in the Southern Rocky Mountains, although the tectonics models for the Laramide orogeny in this region remain controversial (Seager 2004).
Paleozoic to Cenozoic sedimentary rocks are bent and tilted on the flanks of San Juan Uplift and are relatively flat-lying in the central part of the uplifted block. A deep bend on the southern edge of the Laramide uplift in southwestern Colorado created an asymmetrical syncline that defines the San Juan Basin (plate 1). This basin presents about 3,000 feet of structural relief and ~18,000 feet of structural relief between the Needle Mountains Uplift and the deepest part of the San Juan Basin (Kelley 1955). Cather (2004) reported that the evolution of the San Juan Basin involved diachronous subsidence and deposition, with at least three distinct periods of basin development.
The San Juan Basin is bound by abrupt and pronounced structural margins to the northwest (Hogback Monocline), northeast (southwest limb of the Archuleta Anticlinorium), and east (Nacimiento Uplift). In southwestern Colorado, tilted sedimentary strata of the Hogback define a flexure, partly fault-controlled, on the southern and eastern edges of the San Juan Uplift. As uplift of the Needle Mountains block continued into the Paleocene and Eocene, it was dissected by erosion that shed vast amounts of detritus as stream and fan deposits (Ojo Alamo Sandstone, Animas Formation, Nacimiento Formation, and San Jose Formation) into the San Juan Basin. Rock fragments in these deposits show a progressive uncovering of the uplifted block, down to its Proterozoic core. The Animas Formation also contains abundant fragments of volcanic rock, indicating the existence of volcanic centers to the north of the San Juan Basin prior to ~55 Ma.
At about 70 Ma, intermediate to felsic magmas rose into the crust, forming numerous mushroom-shaped bodies called laccoliths along the western edge of Colorado. Eroded remnants of Laramide laccolithic mountains in this part of the world include the La Plata, Abajo, and La Sal Mountains and smaller masses exposed on the fringes of the eastern San Juan Volcanic Field. These Laramide-intrusive complexes form the older component of a belt of Tertiary magmatism that extends from north-central into southwestern Colorado and that provided the loci of many mineralized deposits during the generation and development of the modern Rocky Mountains. Convincing geologic evidence (McCoy et al. 2005a; Tweto and Sims 1963) suggests that this belt, referred to as the Colorado Mineral Belt, has lineage in fractures and cracks that developed during the formation of Proterozoic mountain belts on the North American craton.
The widely accepted model (Dickinson 1981; Dickinson and Snyder 1978; Lipman, Prostka, and Christiansen 1971) for the Laramide orogenic event involves subduction that extended far inland, beneath North America's West Coast. In this "flat-slab," or shallow-subduction, model, the rate of subduction increased over time. This is thought to have caused the subducted slab to become more buoyant and rise to a lower angle beneath the North American Plate. This process caused "scraping" of the subducted plate 1,000 km inboard of the western edge of North America, leading to crustal compression and uplift (Dickinson 1981;Dickinson and Snyder 1978). Gutscher and colleagues (2000) offer convincing evidence for the flat-slab subduction in segments of the active subduction system of western South America. They argue that this process is caused by the subduction of buoyant oceanic plateaus, which allows the subducted slab to be driven up to 800 km from the active trench. Further deformation, up to 200 km inboard, is documented as block-type uplifts, or transpressional faulting for oblique convergence.
Other explanations for the formation of the Laramide Rocky Mountains have been proposed, but they have not provided substantial evidence to gain the support of most of the geosciences community. For example, Gilluly (1971, 1973) proposed that the Laramide orogeny was related to heterogeneities in the crust-mantle structure. Some recent studies provide evidence that forces in the earth at that time might have caused reactivation and movement along old fractures, which facilitated uplift and formed pathways for melted rock (Karlstrom and Humphreys 1998; Karlstrom et al. 2005;McCoy et al. 2005a, 2005b; Mutschler, Larsen, and Bruce 1987). In this model, the need for subduction as the catalyst for uplift and magmatism is less critical because magma production is dependent upon the release of pressure on the mantle to initiate melting rather than on the melting of the subducted oceanic slab. This is further supported by geophysical and geochemical studies that have concluded that the Laramide event did not have a substantial impact on the thermal state of the lithosphere; such an impact would be expected as a result of magmatism related to the proposed low-angle subduction that accompanied mountain building (Karlstrom et al. 2005; Livaccari and Perry 1993; Riter and Smith 1996).
(Continues...)
Excerpted from The Eastern San Juan Mountains by Rob Blair, George Bracksieck. Copyright © 2011 University Press of Colorado. Excerpted by permission of University Press of Colorado.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
Foreword Governor Bill Ritter vii
Preface ix
Acknowledgments xi
Part 1 Physical Environment of the San Juan Mountains
chapter I A Legacy of Mountains Past and Present in the San Juan Region David A. Gonzales Karl E. Karlstrom 3
chapter 2 Tertiary Volcanism in the Eastern San Juan Mountains Peter W. Lipman William C. McIntosh 17
chapter 3 Mineralization in the Eastern San Juan Mountains Philip M. Bethke 39
chapter 4 Geomorphic History of the San Juan Mountains Rob Blair Mary Gillam 61
chapter 5 The Hydrogeology of the San Juan Mountains Jonathan Saul Caine Anna B. Wilson 79
Chapter 6 Long-Terin Temperature Trends in the San Juan Mountains Imtiaz Rangwala James R. Miller 99
Part 2 Biological Communities of the San Juan Mountains
chapter 7 Mountain Lakes and Reservoirs Koren Nydick 113
chapter 8 Fens of the San Juan Mountains Rodney A. Chimner David Cooper 129
chapter 9 Fungi and Lichens of the San Juan Mountains J. Page Lindsey 137
chapter 10 Fire, Climate, and Forest Health Julie E. Korb Rosalind Y. Wu 151
chapter 11 Insects of the San Juans and Effects of Fire on Insect Ecology Deborah Kendall 173
chapter 12 Wildlife of the San Juans: A Story of Abundance and Exploitation Scott Wait Mike Japhet 185
Part 3 Human History of the San Juan Mountains
chapter 13 A Brief Human History of the Eastern San Juan Mountains Andrew Gulliford 203
chapter 14 Disaster in La Garita Mountains Patricia Joy Richmond 213
chapter 15 San Juan Railroading Duane Smith 231
Part 4 Points of Interest in the Eastern San Juan Mountains
chapter 16 Eastern San Juan Mountains Points of Interest Guide Rob Blair Hobie Dixon Kimberlee Miskell-Gerhardt Mary Gillam Scott White 243
Glossary 299
Contributors 311
index 313